1. Field of the Invention
Embodiments of the invention are generally directed to the field of optical systems; more particularly, to an optical surface metrology apparatus and associated method; and, most particularly an optical surface metrology apparatus and associated method for use with a phi (φ)-polynomial surface.
2. Related Art Discussion
Interferometric measuring apparatuses are commonplace in the optical fabrication industry and provide the fabricator with a surface deviation map that can be used to further direct polishing that ensures the surface meets a quality standard required by the optical design. A conventional interferometric apparatus for testing a spherical or planar optical surface includes a coherent light source and set of reference (null) optics that perform two functions: (1) the reference optics create a wavefront that matches the curvature of the surface under test such that the wavefront is retro-reflected back towards the reference optics and, (2) the reference optics create a reference wavefront that will interfere with the wavefront reflected from the surface under test. The interferogram that is generated by the interference of the two wavefronts contains information pertaining to the deviation of the surface under test from the ideal (reference) wavefront. In the case of aspheric and non-rotationally symmetric surfaces, such as, phi-(φ) polynomial surfaces, the reference optics and/or method of measuring the optical surface must be modified to accommodate this new surface type. One type of reference optic that may be employed is a computer generated hologram (CGH), which is a diffractive optical element that encodes an aspheric wavefront into the optical surface such that the light continues to strike the surface under test orthogonally. A CGH reference is accurate within a reasonable range of magnification, but the pricing is prohibitive in many cases, especially if the optical system contains three or more optical surfaces where a separate CGH null is required for each optical surface. Methods that are dynamic, not requiring a separate null for each optic surface, have been introduced in the case of aspheric and off-axis portions of a conic. For these methods the adaptive element is either an actuated, deformable membrane mirror or thin mirror substrate with a bending harness. In each case, the adaptive null is rotationally symmetric and does not provide the degrees of freedom to adequately measure a non-rotationally symmetric optical surface like a phi-polynomial surface.
The inventors recognize that affordable and efficacious solutions to the problems and challenges of testing a nonsymmetric optical surface (such as a phi-polynomial surface, for example), in which the dynamic range of a conventional interferometer apparatus is exceeded would be advantageous and beneficial.